1. Field of the Invention
This invention relates to a method of and an apparatus for controlling boost pressure in a turbocharged internal combustion engine and more particularly to a method of and an apparatus for controlling boost pressure in an internal combustion engine equipped with a turbocharger wherein the timing of boost pressure reduction is determined relative to the ignition timing.
2. Description of the Prior Art
In an internal combustion engine equipped with a turbocharger, it is conventional practice to reduce the boost pressure when the amount of retardation of the ignition timing with respect to the basic ignition timing comes to exceed a predetermined value. This practice is followed, for example, in Japanese Laid-open Patent Application No. 58(1983)-167881.
FIG. 7(a) and FIG. (b) show a comparison of the prior art and the present invention. As seen in FIG. 7(a), in the prior art the boost pressure P2 is reduced when the amount of ignition retardation d.theta.R for prevention of knocking comes to exceed a predetermined reference level. In contrast, in the present invention a decision as to whether or not the boost pressure should be reduced is made based not on the amount of ignition retardation but on the actual ignition timing .theta.ig obtained by subtracting the ignition retardation amount from the basic ignition timing, and when the actual ignition timing .theta.ig exceeds a predetermined reference level the boost pressure is reduced. In this example, when the engine coolant temperature Tw at time t1 is higher than a temperature above which there is danger of damage to the engine, the ignition timing .theta.ig is retarded considerably both in the prior art and in the present invention. In the present invention, however, carrying out a retardation of the ignition timing by an amount large enough to eliminate such an abnormal condition causes the actual ignition timing .theta.ig to immediately exceed the reference level above which the boost pressure is to be reduced so that the boost pressure is immediately reduced at the same time t1 and, as a result, the engine output PS is immediately brought to within the safety limit. On the other hand, in the prior art the boost pressure is not reduced until the amount of retardation d.theta.R reckoned relative to the basic ignition timing comes to exceed the reference level at the time t2. In the meantime, the engine output remains outside the safety limit so that there is a danger of the engine being damaged. Further, as shown in FIG. 7(b), in the prior art the boost pressure is reduced every time the retardation amount d.theta.R exceeds the reference level, even when there is in fact no need for it to be reduced. As a result, unnecessary loss of engine output frequently results. In the present invention, the actual ignition timing .theta.ig does not exceed the basic level under such circumstances and, accordingly, there is no unnecessary reduction in the boost pressure. The problem with the prior art is thus that the decision regarding reduction of the boost pressure is made with reference only to the amount of ignition timing retardation carried out for preventing of knocking, i.e. without reference to changes in ignition timing made necessary by changes in the engine operating condition or environment. As a result, the prior art practice entails a risk of damage to the engine and causes unnecessary reductions in the engine output.